This invention relates to a process and an apparatus for the heat-treatment of fine material which is first preheated in a first zone, subsequently heated in a second zone to the final treatment temperature by passing through a flame, and then separated in a third zone from the hot exhaust gases of the second zone, the hot exhaust gases from the second zone being used for preheating the material in the first zone.
One such process is known for example from our German Pat. No. 1,807,292. This process enables fine material to be calcined at high temperatures in a gas stream over a short distance and, hence, provides for high throughput levels through a calcining or sintering chamber of very simple construction, i.e. with little construction outlay. The most important requirement for uniformly heat-treating all the particles of fine material is that uniformly pre-heated fine material should pass through the zone of highest temperature in a precisely defined, short space of time.
This process has proved to be eminently suitable for heat treating various types of fine material, for example for calcining or sintering cement raw material, lime, dolomite, magnesite, hydrate of alumina, etc. Hitherto, gaseous fuels have been used for the heat treatment in the second zone (i.e. in the high temperature zone).
The object of the present invention is further to develop the process described above to the extent that solid or liquid fuel may also be used for generating the heat required in the second zone.
In the experiments on which the present invention is based, solid or liquid fuel was first directly used in the second zone for producing the flame. It was found that solid or liquid fuel particles frequently settled on the particles of fine material, or were partially absorbed by the particles of fine material (oil droplets for example were sucked up) and, hence, became inaccessible to rapid combustion. The result of this was a relatively high carbon content in the end product which, in many cases (for example in the calcination of alumina hydrate), is undesirable.
If, in the second zone, the fine material passes through a flame produced by solid or liquid fuel and if solid or liquid fuel particles settle on the particles of fine material, the fuel is, in addition, actually burnt to a large extent on the particles of fine material themselves, which results in coking and soot formation and, in many cases, leads to over-calcination of the particles of fine material and to a corresponding reduction in product quality. In some cases, the fine material to be heat-treated even has a catalytic effect upon the cracking process taking place during the combustion of solid or liquid fuel, which promotes coking and, hence, the undesirable increase in the carbon content of the end product.
According to the invention, these disadvantages are obviated by virtue of the fact that the solid or liquid fuel is first gasified or vaporised and the combustible gases formed are used for producing the flame through which the fine material passes in the second zone (high temperature zone). In this way, the fine material is heat treated very uniformly over a short reaction zone, giving an end product of desirably low carbon content. The solid fuel may be gasified or the liquid fuel vaporised by any known process (using externally supplied heat or internally generated heat). Steam may also be used in a known manner for gasifying or vaporisation.
If, after separation, in a third zone from the hot exhaust gases of the second zone, the fine material is cooled in a fourth zone, it is preferable, in order to improve the heat economy of the process as a whole, to use waste air from the cooling zone for vaporising or gasifying the fuel.
As can be seen from the foregoing, the process according to the invention may be used with particular advantage in cases where the end product is not to exceed a certain carbon content, for example in the calcination of alumina hydrate.
However, the process according to the invention may also be used for example in the reduction of finely particulate ore, in which case a deficiency of air sufficient for the reduction process is provided in the second zone (high temperature zone).
In one practical embodiment of the process according to the invention, an adjustable portion of the material separated in the third zone from the hot exhaust gases of the second zone is returned to the second zone where it passes through the flame again. In this way, the individual particles of fine material (on a statistical average) pass repeatedly through the calcining or sintering zone, which further improves the uniformity of the end product. In this respect, it can also be of advantage for the second zone to contain several stages each equipped with a burner through which the fine material successively passes.